The present invention pertains to a gas-measuring device for determining the concentration of a gas by the infrared absorption of the measuring signals sent by a radiation source and received by a radiation detector, and to a gas-measuring process for determining the concentration of a gas by the infrared absorption of the measuring signals sent by an infrared radiation source and received by a quadrant detector comprising two measuring detectors and two reference detectors.
Such gas-measuring devices and gas-measuring processes have been known from various publications and are used, among other things, for monitoring the air quality, detecting air pollutants, for measuring the concentrations of gases, e.g., carbon dioxide, methane, laughing gas, or for determining the alcohol concentration in the breath of a person.
The infrared radiation passing through the gases to be measured is now attenuated by absorption in a gas-specific manner in certain wavelength ranges. The intensity of the absorption is determined by measurement. It depends on the concentration of the gas, the path length of the radiation and other influential factors such as the temperature, the contamination of mirrors and windows or other beam shields, which, not being due to the gas, shall be left out of consideration during the concentration measurement.
An infrared optical gas-measuring device for determining the concentration of a gas comprises, in principle, a radiation source and a radiation detector. The radiation source sends measuring signals in the infrared wavelength range; these measuring signals pass through the gas to be measured, and are partly absorbed by the gas and are received at a reduced intensity by the radiation detector. The reduction in the intensity is an indicator of the gas concentration.
It has been known that two radiation detectors are used in infrared optical devices for compensating intensity reductions of the measuring signals in the infrared wavelength range, which reductions are not due to the gas. While the first detector measures only radiation from a wavelength range in which the gas, whose concentration is to be measured, has an absorption, the second detector is sensitive only to radiation from a spectral range in which the gas does not absorb. The quotient of these two measuring signals changes only if the gas to be detected is present in the gas-measuring path. Aging effects and other changes in the radiation intensity which do not depend on the spectrum normally affect both radiation detectors equally, so that the quotient remains constant in these cases. However, drifts, which are caused by the contamination of the beam path, may occur even in the case of a design with two radiation detectors. Contaminants are, in general, not distributed homogeneously over the beam cross section, so that the spatial radiation intensity distribution over the cross section of the optical system changes. In connection with the remaining asymmetry in the splitting of the radiation between two detectors, this leads to a change in the quotient, i.e., to a drift in the measurement result of the gas-measuring device.
A measuring device of this type has been known from DE 197 13 928 C1. The concentration of gases is determined there by infrared absorption with two identical radiation sources and two radiation detectors, one of which is used as a measuring detector and the other as a reference detector. Due to this and to the use of optical concentrators for bundling the radiation, stable measured values are obtained despite the contamination or the radiation shielding of the optical surfaces exposed to the gases or gas mixtures.
The drawback of these and other infrared optical gas-measuring devices of this type is the large amount of material needed and the large space requirement as well as the great effort needed for adjustment as a consequence of the mechanical tolerances of the optical requirements.
A multispectral sensor with which different spectral ranges of a radiation to be measured are determined has been known from DE 41 33 481 C2. The multispectral sensor is characterized by small dimensions and high reliability of operation. These advantageous properties are guaranteed by a highly reflecting optical beam-splitting device, which splits the light via a plurality of filters of different spectral transmission ranges among radiation-sensitive elements arranged behind them.
The drawback of this multispectral sensor is that it has no precautionary measures to compensate non-gas-related changes in intensity. Distortions of the measured results cannot therefore be ruled out.
The basic object of the present invention is therefore to provide a compact infrared optical gas-measuring device and a gas-measuring process which furnish reliable measured results for the concentration determination despite disturbances occurring in the path of the radiation.
According to the invention a gas-measuring device is provided for determining the concentration of a gas by the infrared absorption of the measuring signals sent by a radiation source and received by a radiation detector. The radiation source is a infrared radiation source. The radiation detector is a quadrant detector, which comprises four individual detectors arranged in a square pattern. A pyramid-like beam splitter divides the measuring signals of the infrared radiation source among the four individual detectors. The four individual detectors comprise two measuring detectors located opposite one another and two reference detectors located opposite one another. The measuring detectors are equipped with first identical infrared filters and the reference detectors are equipped with second identical infrared filters.
The gas-measuring process for determining the concentration of a gas by the infrared absorption of the measuring signals sent by an infrared radiation source and received by the quadrant detector comprising two measuring detectors and two reference detectors includes forming the quotients:       Q1    =          M1      R1        ,      Q2    =          M1      R2        ,      Q3    =          M2      R1        ,      Q4    =          M2      R2        ,
from the measured signals M1, M2 received by the measuring detectors and the measuring signals R1, R2 received at the reference detectors and using the formed quotients to determine the gas concentration.
An essential advantage of the gas-measuring device according to the present invention is that due to the use of a pyramid-like beam splitter, the measuring signals of an infrared radiation source are divided among four individual detectors, doing so in a compact design with only few components. As a result, the gas-measuring device can be accommodated in a smaller housing. This leads to increased stability with respect to thermal or mechanical stress and is advantageous under real conditions of use when the gas-measuring device is subject to great temperature variations or shocks.
Another advantage arises from the measuring signals received by the four individual detectors. The evaluation of four signals guarantees higher reliability than that of two signals during the measurement of the gas concentration because of a lower probability of false alarms due to the exceeding of preset concentration values when optical components are in reality only partially contaminated or partial components of the gas-measuring device fail.
It is advantageous in the gas-measuring process according to the present invention that four measuring signals are evaluated and that spatially inhomogeneous reductions in the radiation intensity can be detected and are thus taken duly into account for the determination of the gas concentration.
The present invention will be described below as an example based on the schematic drawings.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.